1. Field of the Invention
This invention relates to a micro-accelerometer made by micromachining techniques and used to detect earthquakes. The invention also relates to a preloaded linear beam vibration sensor (i.e. an unbalanced see-saw type vibration sensor) made by micromachining techniques.
2. Description of the Prior Art
Individual terminals of a gas supply network must be shut quickly and reliably in the event of large-scale earthquakes. For this purpose, an earthquake sensor is equipped with each user's terminal of the network to detect earthquakes and to shut the terminal in response to a detection signal.
Existing accelerometers for detecting an earthquake can be classified into two types: one of a broad band type using a small vibrator and capable of detecting vibrations over broad bands, and the other of a narrow band type whose frequency characteristics are limited to low frequencies so as to detect only earthquake frequencies. These accelerometers have arrangements in which a mass is connected to a frame via a spring so as to detect acceleration in terms of changes in relative position between the mass and the frame.
In order to implement a linear accelerometer with a low natural frequency to be sensitive only to low frequencies; i.e. vibrations contained in earthquakes, the accelerometer needs to be large. For instance, to detect an acceleration of 100 Gal below 1 Hz with an accelerometer having a natural frequency of 1 Hz, a displacement of the mass-spring system in the accelerometer of 2.5 cm is required. For allowing such a large displacement in a linear system, its vibrator must be, at least, as long as the length of the displacement, and cannot take advantage of using micromachining techniques to fabricate the accelerometer.
Accelerometers for detecting earthquakes should ignore accelerations of frequencies above 5 Hz in order to eliminate influences of noise. Existing wide band accelerometers have very high band resonant frequencies, typically 1000 Hz or more. If such a wide band accelerometer is used to detect earthquake waves, then it may erroneously recognize traffic or other vibrations as earthquakes. In order to overcome the problem, they need an electric filter for removing high frequency components from detection signals so as to extract only earthquake signals.
FIG. 15 shows a basic structure of an earthquake sensor currently used in gas meters. The earthquake sensor 100 uses a steel ball 150 housed in a container 110 having a funnel-like cavity 130 in the center of its bottom 120. When an acceleration is applied to the container 110, the steel ball 150 runs up the slope 131 of the cavity 130, and touches an electrode 141 provided on the circumferential wall 140 of the container 110, thus short-circuiting the electrode 141 and another electrode 121 at the bottom, causing a signal indicating application of an acceleration above a predetermined value to be exerted.
This acceleration sensor exhibits the characteristics shown in FIG. 16 indicating acceleration (Gal) on the ordinate and displacement (mm) on the abscissa. That is, when the inner diameter of the housing 110 is 21.6 mm, the diameter of the steel ball 150 is 15.9 mm, and the angle .alpha. of the slope of the cavity 130 is 6.42 degrees, the steel ball moves by 2.477 mm at the acceleration of (5/7).multidot.M.multidot.g.multidot.sin.alpha..multidot.cos and causes a detection signal to be exerted. In the equation, M is the mass of ball, and g is the gravitational acceleration.
An accelerometer used as a seismograph requires sensitivity to accelerations of 85 to 150 Gal for frequencies between 1 and 5 Hz, and requires a roll-off (decrease in sensitivity) of approximately 60 dB per decade of frequencies above 5 Hz.
In order that a detector element does not largely deform when an earthquake acceleration is applied, the detector element must be small, and the use of micromachining techniques to make such accelerometers would be advantageous. In addition, such accelerometers themselves for detecting earthquake waves must have a narrow detection bandwidth of approximately 1 to 5 Hz to eliminate the need for an expensive low pass electric filter.